Humidity determination



Aug. 19, 1958 D. R. B LUMER HUMIDITY DETERMINATION Filed Sept. 23, 1954DESSICANT m T N w R we M U L B R D L W D ATTORNEY United States PatentHUMIDITY DETERMINATION Donald R. Blumer, St. Paul, Minn., assignor toMinneapolis-Honeywell Regulator Company, Minneapolis, Minn., acorporation of Delaware Application September 23, 1954, Serial No.457,912

2 Claims. (Cl. 23-232) The present invention relates generally to thedetermination of humidity or water vapor in gaseous mixtures. Morespecifically, the invention relates to the determination of humidity ina gaseous mixture by means of comparing the thermal conductivity of areference gas with that of an unknown gas wherein the water vapor in theunknown gas has previously been converted to hydrogen. Previously,attempts have been made to measure the water vapor content of air bymeans of comparing the thermal conductivity of an unknown sample withthat of a sample of either desiccated or saturated air; however, thesesystems have not been entirely satisfactory because of the lack ofsensitivity of this type of device. It is noted, in this connection,that the thermal conductivity of air at 32 is 0.0140 B. t. u./hr. sq.ft. F. ft. while the same thermal conductivity for water vapor at thattemperature is about 0.0110. On the other hand, with my apparatus, thewater vapor is converted to hydrogen which has a thermal conductivitycoefficient of 0.100. It is seen, therefore, that the thermalconductivity of hydrogen is substantially one order of magnitude greaterthan that of either air or water vapor, and hence the sensitivity of myapparatus is vely high.

Therefore, it is an object of the present invention to provide a methodand apparatus for determining the water vapor content of a gaseousmixture by means of a thermal conductivity bridge wherein the watervapor in the unknown sample has previously been converted to hydrogen.

It is a further object of the present invention to provide an improvedmethod and apparatus for determining water vapor content of gaseousmixtures with a particularly high degree of sensitivity. It is still afurther object of the present invention to provide an improved methodand device for determining the water vapor content of air by means ofthermal conductivity comparisons utilizing a sample of air which has hadits water vapor content converted to molecular hydrogen prior to thethermal conductivity comparisons.

In accordance with the present invention, therefore, there is provided,for example, a Wheatstone bridge type of device in which two oppositelegs are provided with chambers for passing gas samples therethrough andwherein the relative ratios of thermal conductivities between the gasesbeing passed through the two chambers may be determined. In order toconvert the water vapor con tained in the gas sample undergoingdetermination to molecular hydrogen, there is provided in the lineupstream from the thermal conductivity chamber, a container which hasavailable an active metal hydride which reacts quantitatively with thewater vapor to release hydrogen. For example, calcium hydride has beenfound satisfactory in this connection, and the reaction is carried on asillustrated in the following formula:

2,848,306 Patented Aug. 19, 1958 or, preferably 02.11 111,0 Ca0 211: T

If desired for extra sensitivity, the other thermal conductivity sensingleg of the bridge, which uses a standard or reference gas, may beprovided with a dessicating chamber or the like in order to remove thewater vapor from the reference gas, thereby providing a consistentstandard or reference sample. If, on the other hand, this particularlyhigh degree of sensitivity is not necessarily required, the two chambersmay be connected in series, with the metal hydride chamber positionedbetween the two cells. Of course, a source of electrical energy isrequired for the device as well as an indicating meter, such as agalvanometer or the like connected across opposite legs of the bridgenetwork. The gas may be moved through the system by means of a suitablepower source such as an impeller or the like.

My invention may be more easily and fully comprehended with reference tothe accompanying drawings in which:

Figure l is a schematic view showing a preferred modi fication of thepresent invention;

Figure 2 is also a schematic view of a slightly modified form of thepresent invention; and

Figure 3 is a detailed view, on a slightly enlarged scale and partiallyin section showing a thermal conductivity heat 350C.

cell which may be utilized in connection with the bridge networks asshown.

In accordance with the preferred modification of the present invention,there is shown in Figure l a Wheatstone bridge system generallydesignated 10 which includes a pair of standard resistance members 11and 12 and a pair of thermal conductivity resistance chambers 13 and 14.There is also provided in this system a source of potential 16controlled by the switch 15, and an indicator 17 for indicating thedegree of electrical unbalance present in the system. The source 16 isshown as a unidirectional power source such as the battery supply 16. Inorder to provide for air travel through the thermal conductivitymeasuring chambers 13 and 14, there are provided impellers or fans 18and 19 respectively oper-' ated from any suitable source of power, notshown. A chamber 20 containing a metal hydride is situated in the feedline 21 which leads to the thermal conductivity chamber 13. Likewise, onthe other leg of the bridge, wherein chamber 14 is situated, a desiccantmedium 23 is placed in the line 24 which feeds the reference gas, inthis case, dry air, to the thermal conductivity chamber 14. Flowregulators should be provided for the impellers 18 and 19 in order thatvsubstantially equal quantities of gas will flow through each of thesystems.

In order to operate the device, switch 15 is closed in order to apply apotential across the opposite legs of the bridge system, and theindicating meter, such as the galvanometer 17, is adjusted to a suitablezero position with the same gas passing through the separate cells. Uponsatisfactory adjustment of the zero point, impellers 18 and 19 are setinto motion, thereby drawing respective samples of gas across the heatedfilaments 13A and 14A which are situated in the interior of the thermalconductivity chambers 13 and 14 respectively. These filaments arepreferably constructed of any type of resistor material which has arelatively high coefiicient of change with temperature, such asplatinum, nickel, or any suitable thermistor material. The gas samplewhich enters line 21 in the direction of the arrow 25 passes over a bedof metal hydride in the chamber 20 and up through conduit 21 to thethermal conductivity chamber 13, passing in contact with the resistor orthermistor 13A, and finally being exhausted through the impeller,

18 in the direction of arrow 26. On the other leg of the bridge, astandard or reference gas, which may, for convenience, he the same airwhich is undergoing analysis in the opposite leg of the bridge isintroduced into the line 24 in the direction of the arrow 28. This gasthen preferably: passes over a desiccant bed, which may be for examplemagnesium perchlorate, silica gel, phosphorous pentoxide, or the like,and then moves up the conduit 24 and through the thermal conductivitychamber 14, moving over and across the resistor element or thermistor14A and finally to the impeller 19 and out of the system as is indicatedby the arrow 29. Assuming the air undergoing test is moist, when thesamples pass through the chambers 13 and 14, the resistor 13A is cooledto a greater extent than is the resistor 14A due to the higher thermalconductivity of hydrogen. The conversion of the water vapor to hydrogenis substantially quantitative, and therefore the greater the proportionof water vapor present in the gas being sampled, the greater will be thehydrogen content of the gas passing through the conductivity cells. Inthis connection, the greater the proportion of hydrogen in the gaspassing through the cell, the greater will be the cooling eifect of thegas on the resistor included in the cell. This cooling effect, ofcourse, may be read ofl the indicating means in terms of a degree ofunbalancein the bridge. Due to the temperature sensitivity of resistanceof the respective resistance elements, an unbalance is then obtainedacross opposite legs of the bridge network and the magnitude of thisunbalance is indicated by the galvanometer 17. For convenience, it willof course, be possible to calibrate the meter or galvanometer 17directly in percent of absolute humidity present in the gas systemundergoing test.

Attention is now directed to Figure 2 wherein there is shown anothermodification of the present invention. In this connection, there isprovided a bridge system generally designated which includes a pair ofstandard resistor members 31 and 32, a pair of thermal conductivitymeasuring chambers 34 and 35 which house temperature responsive resistormembers 34A and 35A respectively. There is also provided a source ofpotential 36 which is controlled by the switch 37, and a suitable meter38 for measuring unbalance of the system. An impeller or fan 39 isprovided for drawing gas through the sampling system, and is driven byany suitable source of power, not shown. A conduit system 40 is providedfor moving the gas samples through the system. There is further provideda metal hydride chamber .42 which contains a suitable metal hydride,which will quantitatively convert water vapor in the sample to molecularhydrogen. Therefore, in a given sample moving through the system, thethermal conductivity of the raw air is measured in the chamber 35 and isthence converted to a mixture of dry air and hydrogen by the metalhydride chamber 42, and this converted sample is then passed through thethermal conductivity measuring cell 34 and finally is exhausted from thesystem by means of the impeller 39 in the direction of the arrow 44.

The operation of the modification as illustrated in Figure 2 issubstantially the same as that of the device illustrated in Figure l.The only distinction in the two systems is that the reference sample inthe device of Figure 1 represents a more standard material, such as dryair as opposed to wet air which is utilized in the device illustrated inFigure 2.

Attention is now directed to Figure 3 wherein there is shown on aslightly enlarged scale a thermal conductivity measuring cell generallydesignated and which includes a housing 46 of suitable thermalconductivity material, such as brass, stainless steel or the like. Thesecells are preferably formed in a single block and thus a substantiallyconstant temperature is maintained. The cell 45 is provided with anelectrical resistance element 47 which is sensitive in its resistancecharacteristics to changes in ambient temperature. The resistor 47 issealed into the chamber 45 by the plugs 48 and 49 which are electricalresistors and preferably moisture repellant. In order to pass a gassample through this chamber, there are provided ports 50 situated onopposite sides of the chamber. Thus, in operation, a gas sample passesover a substantial portion of the resistor member 47 included in thecell 45 and the influence of the thermal conductivity of the gas passingover the resistor 47 may be read from a suitable indicating member aspreviously shown included Within a bridge arrangement.

In addition to calcium hydride, lithium hydride may be satisfactorilyutilized in connection with the present invention particularly in thepresence of inert gases. It will be noted, however, that with lithiumhydride one molecule of hydrogen is liberated for each molecule of watercontacted. This material reacts according to the following equation:

Of course, it will be appreciated that various other hydride materialwhich exchange hydrogen for water quantitatively may be satisfactorilyutilized in connection with the present device such, for example, asbarium hydride and similar commercially available hydrides. In specificcases some of the more active hydrides such as lithium aluminum hydrideand the like may be used such as with inert gases such as nitrogen,argon, or the like, since in the presence of air they are likely to heatup suthciently to catch fire by reaction with the oxygen of the air.

In addition to the two cell bridge system illustrated in Figures 1 and2, multicell systems may be used, particularly a four cell system inwhich opposite arms of the bridge are exposed to the two gases of thesame composition for each pair, thereby increasing the electricalsensitivity of the bridge. Similarly, an eight cell bridge which isappropriately connected to the non-hydrogen and hydrogen bearing gasstreams may be used to increase the sensitivity still further. One mayamplify the quantity of hydrogen present in the sample passing throughthe cells if greater sensitivity is desired. In this connection, the gasafter passing over the hydride bed is passed over or through aplatiniyed or palladiycd silica gel or asbestos layer or similarcatalyst at a suitable temperature wherein the hydrogen present combineswith oxygen from the air to form Water vapor. This gas is then passedthrough or over a second hydride bed and hydrogen is formed according tothe equations 2H 0 ZH O Therefore, the quantity of hydrogen available tothe cells is doubled. Of course, this procedure may be repeated todouble the hydrogen available at each stage. The thermal eitect isactually more than doubled since a portion of the low thermalconductivity oxygen is removed from the system each time the gas ispassed through the catalyst bed after passing over the hydride bed.

Although various specific embodiments of the invention herein have beendisclosed, it will be understood that there is no invention to limit thescope of the present invention to these specific embodiments alone,since they are used for purposes of illustration only. Many details ofcomposition and procedure may be varied without departing from theprinciples of this invention. It is therefore not my purpose to limitthe patent granted on this application otherwise than necessitated bythe scope of the appended claims.

I claim as my invention:

1. The method of determining the water vapor content of a gaseousmixture which includes passing a standard reference gas through a firstchamber wherein its relative thermal conductivity may be measured,passing a sample of a gas of unknown composition through a metal hydridebed wherein the water vapor is converted to molecular hydrogen, andthence passing said converted gas through a second thermal conductivitychamber wherein its thermal conductivity may be measured and comparedwith that of the reference gas.

2. The method of determining the water vapor content of a gaseousmixture which includes passing said mixture through a zone wherein watervapor is converted 6 References Cited in the file of this patent UNITEDSTATES PATENTS Schneider Apr. 26, 1932 OTHER REFERENCES Harris et al.:Analytical Chemistry, vol. 23, No. 5, May 1951, pages 736-9.

Daynes: Gas Analysis by Measurements of Thermal to molecular hydrogen,then to combining the hydrogen 10 Conductivity, Cambridge UniversityPress, London thus liberated with oxygen to form water vapor, thenpassing said gas through a second zone wherein the water vapor presentis converted to hydrogen, and finally passing said converted gas througha zone wherein its hydrogen content is determined relative to areference gas. 15

Technologic Papers of the Bureau of Standards, No. 249, ThermalConductivity Method in the Analysis of Gases, January 7, 1924, page 49.

Chemical Abstract, vol. 34, column 3624 (1940).

1. THE METHOD OF DETERMINING THE EATER VAPOR CONTENT OF A GASEOUSMIXTURE WHICH INCLUDES PASSING A STANDARD REFERENCE GAS THROUGH A FIRSTCHAMBER WHEREIN ITS RELATIVE THERMAL CONDUCTIVITY MAY BE MEASURED,PASSING A SAMPLE OF A GAS OF UNKNOWN COMPOSITION THROUGH A METAL HYDRIDEBED WHEREIN THE WATER VAPOR IS CONVERTED TO MOLECULAR HYDROGEN, ANDTHENCE PASSING SAID CONVERTED GAS THROUGH A SECOND THERMAL CONDUCTIVITYCHAMBER WHEREIN ITS THERMAL CONDUCTIVITY MAY BE MEASURED AND COMPAREDWITH THAT OF THE REFERENCE GAS.